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Cardiovascular responses to sustained isometric work in a hot environment Iacobellis, Joseph


The purpose of this study was to investigate the changes in cardiovascular dynamics as depicted by systolic time intervals, blood pressure and heart rate during a 50% MVC and 100% MVC isometric contraction of the forearm in a control and heated environment. Fourteen normal male volunteers aged 20 to 31 were used as subjects. Simultaneous recordings of the phonocardiogram, electrocardiogram, carotid pulse wave and blood pressure were conducted for each subject at rest and during exercise in a seated position. Subjects were tested, in room temperature and in a sauna where the skin temperature was raised to 40°C -41°C. Testing took place on two separate days with one day of rest in between. Half of the subjects experienced the heated conditions first, while the other half was tested in room conditions first. The results from two of the subjects were discarded because of poor quality reproduction of the time interval recordings. For each recording only the three clearest cycles closest to the termination of the contraction period were used for statistical analysis. The data were treated with a two-way ANOVA for each dependent variable. In some cases a post-hoc analysis (Newman-Keuls method) was used to determine specific differences between workload or environment effects. The fourteen dependent variables studied were divided into the following groups: a) Systole related variables left ventricular ejection time (LVET) mechanical systole (MS) total systole (TS) ejection time index (ETI) Diastole related variables cycle time (CT) diastole (DIAS) Sympathoadrenergic Activity (Contractility) pre-ejection period (PEP) isovolumetric contraction period (ICP) PEP/LVET (ratio) Afterload systolic blood pressure (BPs) diastolic blood pressure (BPd) Electromechanical Lag (EML) Heart Rate (HR) Myocardial Oxygen Consumption (Index) triple product (TRIP) -CONCLUSIONS The oxygen consumption of the myocardium as depicted by the triple product significantly increased during submaximal and maximal isometric handgrip contraction. This increase was evident at room temperature and during body heating. There was no significant change in the myocardial oxygen consumption as depicted by TRIP at rest or during isometric forearm contraction between the control and heated environments. This suggests that the heat stress did not significantly increase the myocardial oxygen requirements. 3. In a state of rest, increasing the skin temperature to between 40°C -41°C did not significantly alter either BPs or BPd when compared to a resting state at room temperature. However, BPs and BPd were substantially lower during isometric work in the heat than during isometric work at room temperature. 4. BPs and BPd significantly increased during 50% MVC and 100% MVC static contractions of the forearm. This increase was demonstrated in both environmental conditions. 5. All variables depicting changes in left ventricular systole (LVET; MS; TS) and ventricular diastole (diastole and CT) were found to become significantly reduced with submaximal and maximal static contractions of the forearm. These changes were evident in both environments. 6. A strong inverse correlation was found between HR and LVET, CT and diastole. HR significantly increased from rest to 100% MVC in both environmental conditions. Consequently, it is suggested that alterations in LVET, CT and diastole are largely determined by the rate of myocardial contraction. 7. The ejection time index significantly increased in both environmental conditions with a 50% MVC and 100% MVC static contraction of the forearm. The electromechanical lag showed a general tendency to decrease during an isometric handgrip contraction. However, subsequent post-hoc analysis (Newman-Keuls) demonstrated that EML did not significantly decrease during a submaximal or maximal isometric contraction of the forearm. It is suggested that care be taken to choose a proper statistical procedure for analysis of EML. The contractility of the heart as depicted by changes in LCP, PEP and PEP/LVET increases in response to a pressure load produced by static exertion but is not significantly altered by an augmented volume load associated with heat stress. HR, LVET, MS, TS, PEP, ICP, PEP/LVET and EML changed in an additive fashion from rest to 100% MVC during subjection to a volume load and pressure load simultaneously. In contrast alterations in BPs, BPd, ETI, CT, diastole, and TRIP displayed interactive characteristics during the same test conditions .

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